Obesity and diabetes have reached epidemic level in the United States. Diabetes and its associated complications are becoming one of the leading causes of morbidity In this country. Although several hypotheses link obesity and Insulin resistance, the role of circadian rhythm in the pathogenesis of diabetes has not been well addressed. Interestingly, there Is a clear circadlan variation In fasting glucose, Insulin sensitivity and glucose tolerance, which Is severely dampened In the condition of obesity and diabetes. The orphan nuclear receptor Rev-erba emerged as a core circadian gene. By repressing gene expression of another clock gene Bmal1, Rev-erba functions as a negative regulator of circadian rhythm. We showed GSK3p-dependent phosphorylation is required for maintaining the protein stability of Rev-erba and is crucial for synchronizing the circadlan oscillation of Email gene In vitro. To extend our knowledge on GSK3pdependent regulation of Rev-erba, we propose two specific aims in the first phase of this application (K99 phase), including: aim 1. Elucidate the signal patliways upstream of GSKSp mediating Rev-erba degradation in hepatocytes;aim 2. Determine in vivo roles of Rev-erba phosphorylation by GSKSp in circadian rhythm and metabolism. A mouse model expressing Rev-erba mutant which mimics GSK3P phosphorylation In liver will be created to address these questions. This will be the first analysis to detemnine the role of Rev-erba as a clock protein In liver circadlan rhythm and glucose metabolism. This study will be carried out in the Penn Diabetes Center at University of Pennsylvania under the supervision of Dr. Mitchell Lazar. The center has expertise in diabetes and obesity research, and will provide outstanding environment to conduct the proposed project. Meanwhile, the project will provide superb training for the principle Investigator, Dr. Lel Yin, to develop academic career In the field of diabetes and obesity. The role of the ubiquitln-proteasome pathway (UPP) has not been well-studied In the process of metabolism and energy homeostasis. We recently discovered that Cullln 4A-based E3 ligase regulates protein stability of Rev-erba as well as gene expression Important for gluconeogenesis, suggesting Cullln 4A is a novel modulator functioning in both circadlan rhythm and metabolism. Therefore, the specific aim 3 proposed during the ROO phase will be focused on unraveling roles of the Cullin 4A E3 ubiquitin ligase in regulating circadian rhythm, insulin signaling and glucose metabolism. We expect this work will provide a panel of evidences for the functional Importance of the ubiqultin-proteaspme system in the regulation of metabolism. In addition, this work may shed the new light on Identifying the new therapeutic targets for treating Insulin resistance and diabetes.